Superalloy jet engine components are exposed to extreme operating conditions that can deleteriously affect the surface thereof. In order to protect the surface, a sacrificial intermetallic layer is applied to the surface and forms a protective oxide layer while the jet engine component is in use. After the sacrificial intermetallic layer has worn thin during use, it is removed and a new sacrificial intermetallic layer is provided. This process is repeated as many times as possible to prolong the useful life of the jet engine component.
The intermetallic layer is commonly provided by a simple chemical vapor deposition (CVD) process in which the cleaned jet engine component is exposed to an oxygen-depleted environment of a reaction chamber. Inside the reaction chamber is typically an activator material and a donor material including at least one metal to be integrated into the intermetallic layer. The reaction chamber is purged of atmospheric gases and evacuated. The activator material and donor material are heated to generate vapor phase reactants that cause metal to be transported from the donor material to the jet engine component. The intermetallic layer formed on the jet engine component may include intrinsic metal diffused outwardly from the alloy forming the jet engine component. However, the intermetallic layer must also include at least one extrinsic metal originating from the donor material.
The most common extrinsic metal used in the intermetallic layer is aluminum. To that end, typical donor materials include aluminum such that aluminum forms the bulk of the intermetallic layer. It is desired, however, that there also be a meaningful concentration of a second, extrinsic metal in the intermetallic layer. Current techniques for integrating a second, extrinsic metal into the intermetallic layer are costly and cumbersome, and often result in less than desirable intermetallic layers.
For example, typical donor material may comprise powder or chunklets of a chromium aluminum alloy in which chromium is present in the alloy for raising the material's melting point. During the CVD process, it is believed that little if any of the chromium will actually be transported from the donor material to the jet engine component and, even if the chromium reaches the jet engine component, it may be incorporated into the intermetallic layer in a non-uniform manner. Indeed, as the aluminum is more chemically active than chromium, chromium from the donor material may not integrate in a significant concentration into the intermetallic layer until the aluminum from the donor material is exhausted. The result can be intermetallic layers characterized by inferior properties and/or unduly long cycle times. In certain applications, the inability to release chromium from the donor material may prevent the achievement of beneficial effects derived from the presence of chromium in the coating forming on the jet engine component.
Other approaches for integrating two extrinsic metals into the intermetallic layer suffer from comparable drawbacks. By way of example, it has been proposed to first coat the jet engine component with chromium or platinum by, for example, electroplating, before placing it into the reaction chamber of the simple CVD system and forming the intermetallic layer. While the result can be intermetallic layers containing chromium or platinum and aluminum, the latter originating from the donor material, the process involved is quite time-consuming and costly, and also uses significantly more chromium or platinum than required. Coatings formed by electroplating also suffer from non-uniformity due to the natural tendencies of electroplated coatings to be thicker at and near sharp edges.
Accordingly, there is a need for an improved CVD apparatus and method for applying an aluminide layer having two extrinsic metals to superalloy jet engine components and other types of superalloy components.